1. Field of Invention
This invention relates to electro-optic cells and how to configure and fabricate electro-optic cells so as to enhance performance attainable in a variety of applications including image forming systems.
2. Prior Art
As well understood by those knowledgeable in the state of the art, the cell gap of a liquid crystal cell influences several performance characteristics related to cell operation. Display applications involving liquid crystal cells typically require the cells to possess a uniformly thin, flat layer of liquid crystal material sealed in a suitable environment in order to produce an image of satisfactory quality. Techniques utilized in the prior art to establish a uniformly thin flat layer of a liquid crystal material include the use of a gasket sandwhiched between a pair of opposing substrates. Utilizing a pair of substrates to overlap opposing surfaces of the gasket provides the necessary mechanical support to fix the cell gap to a value dependent upon the thickness of the gasket.
When utilizing a gasket to establish a cell gap, the gasket is limited to the periphery of the cell so as to avoid obstructing a wavefront interacting with the flat layer of liquid crystal material. Gaskets can therefore be classified as a hollow element. Due to the thinness requirement required from gaskets, gaskets lack rigidity. Examples of prior art electro optic cells which utilized gaskets include U.S. Pat. No. 3,862,830 to Stern, Jan. 28, 1975 and U.S. Pat. No. 3,781,087 to Nagasaki, Dec. 25, 1973. In U.S. Pat. No. 3,862,830, the gasket is designated as element 19 in FIGS. 1 and 2 and described in column 2, lines 6 thru 8. In U.S. Pat. No. 3,781,087 a gasket denoted as element 12 in FIG. 1 and is utilized to separate surfaces of opposing substrates.
As identified in the patent application submitted by Craig D. Engle, titled Electron Beam Addressed Crystal Cell, Ser. No. 08/699,511, filing date Aug. 19, 1996 electro-optic cells which utilize a gasket to influence the cell gap of a liquid crystal cell suffer from several complications which adversely effect cell performance. Undesirable effects associated with the use of gaskets include variations in the cell gap from cell to cell. This is attributed to the complications associated with manufacturing thin, flat gaskets.
As identified in the above cited patent application, gaskets are an example of a spacer technique which is a member of a broader class of spacer technology. The terminology "discrete spacer technology" was introduced in application Ser. No. 08/699,511 to describe components which assist in establishing a cell gap between a pair of opposing substrates of a cell and which are permanently added to the cell, but are not considered an integral part of either of the substrates. The language "discrete spacer technology" emphasizes the fundamental differences which exist between the prior art and the techniques identified in patent application Ser. No. 08/699,511 for establishing a cell gap.
Additional examples of the prior art which utilized a gasket to bound a hollow region existing between opposing substrates includes U.S. Pat. No. 4,443,063 to Mishiyama, Apr. 17, 1984. A gasket denoted as sealing element 10 is illustrated in FIG. 2, and FIGS. 4 thru 6. As is readily apparent from all relevant figures, sealing element 10 is sandwhiched between surfaces of an opposing pair of substrates. Accordingly, sealing element 10 is a discrete hollow spacer and the electro-optic cell of U.S. Pat. No. 4,443,063 will be afflicted by the undesirable aspects associated therewith.
As further identified in patent application Ser. No. 08/699,511, the use of microspheres distributed within the cell gap are a second example of discrete spacer technology. Use of microspheres distributed within the cell gap of a liquid crystal cell adversely effects image quality of a magnified image of the cell. Consequently, the use of microspheres with projection display devices is deemed undesirable. Examples of the prior art which utilized microspheres include U.S. Pat. No. 4,784,883, Nov. 15, 1988, to Chitwood et al.
Prior art devices exist which do not utilize discrete spacer technology to produce a uniformly thin flat layer of liquid crystal material. Such techniques include depositing a layer comprising a liquid crystal material on the single substrate. See for instance information contained in U.S. Pat. No. 3,409,404, Nov. 5, 1968 to Fergason and the article titled Liquid Crystal Media for Electron Beam Recording, by J. R. Hansen and R. J. Schneeberger, IEEE Transactions on Electron Devices, Vol. ED-15, No. 11, November 1968. The approaches described in the above references suffer from the fact that the thickness of the flat layer of liquid crystal material is dependent upon the properties of the liquid crystal material and/or the liquid crystal is not provided a hermetically sealed environment. Such limitations could require a sequence of operations which are incompatible with the implementation of various addressing mechanisms necessary to operate the cell.
Limitations of discrete spacer technology are noted in U.S. Pat. No. 4,166,544 to Soref, Sep. 26, 1978. U.S. Pat. No. 4,116,544 identifies a spacer technique comprising a plurality of mesas formed on a substrate. Although mesas improves the control over the separation gap in liquid crystal cells as compared to the more conventional gasket, such an approach requires relatively complicated processing steps to implement.
U.S. Pat. No. 5,016,987 to Smith Jr. May 21, 1991 describes an approach which involves the use of three substrates to seal a uniformly thin flat layer of liquid crystal material. U.S. Pat. No. 5,016,987 requires a large number of complementary surfaces to be fabricated on each of the three substrates and as a consequence, is a relatively complicated device to fabricate. This is attributed to the requirement that each of the opposing pair of substrates are inserted within an deformable annulus which contains a chevron profile.
Additional applications which utilize liquid crystal cells include optical focusing systems. See for instance U.S. Pat. No. 4,037,929 to Bricot et al, Jul. 26, 1977, and U.S. Pat. No. 4,190,330 to Berreman, Feb. 26, 1980. Applications which involve a cell functioning as a lens require variations in the optical path length of a wavefront which traverses the cell to be a well behaved function of the cell's aperture. Consequently, the nature of the addressing mechanism which applies electric fields to the cell and/or the cell shape must be implemented in a manner to accommodate such a functional relationship. See for instance information in U.S. Pat. No. 4,190,330, column 4, lines 12 to lines 22.
Applications which involve cells functioning as spatial light modulators and form an image of the uniformly thin, flat layer of liquid crystal material impose no such constraints. Accordingly, a need exists for a technique which establishes a uniformly thin, flat cell gap in electro-optic cells and which eliminates the undesirable aspects associated with the use of discrete spacer technology.